(1) Field of the Invention
The present invention relates to a moving picture conversion apparatus that converts encoded moving picture code strings, and in particular to improving efficiency of conversion to moving picture code strings encoded with a different motion compensated prediction method.
(2) Description of the Related Art
With advances in digitization of moving pictures in recent years, moving pictures are now able to be transmitted over the Internet and the like as digital data, and recorded in storage media.
The size of such moving picture data has escalated in order to meet demands with respect to color, resolution, and smoothness of motion of moving pictures, and while the speed of communication networks and the capacity of storage media have increased, techniques to compress moving picture data are still necessary.
Encoding compression methods such as the MPEG-2 standard and the MPEG-4 standard have been developed in response to such needs. The MPEG-2 standard aims to implement high picture quality in television broadcasting ranging from existing television through to high-vision television, while the MPEG-4 standard aims to implement a high compression rate for mobile and Internet usage and the like.
Although these two standards have different uses in accordance with their respective features, it is desirable from the user's point of view that moving pictures are able to be converted between the standards. As one example, if a television program recorded in MPEG-2 is converted to MPEG-4 data, the television program can be viewed in a mobile playback apparatus.
Ordinarily, such data conversion is carried out by completely decoding pre-conversion data such as data encoded according to MPEG-2, and then re-encoding the decoded moving pictures in a post-conversion format such as MPEG-4.
This conversion method, while having the advantage of being able to be implemented by simply connecting a conventional encoder and decoder, is considerably inconvenient for a user who is accustomed to high-speed dubbing from an HDD (hard disk drive) to a DVD (digital versatile disk), due to the fact that the time taken for conversion is essentially the same as the playback time of the moving picture.
In view of this problem, the time taken for such data conversion is reduced by reusing part of the pre-conversion encoded data.
This conversion method works on the assumption that pre-conversion encoding processing and post-conversion encoding processing are essentially the same. For instance, motion vectors may be reused in conversion of data from a MPEG-2 to MPEG-4, which is the higher standard, in order to cut the time required for motion vector search processing, and consequently reduce the conversion time significantly.
However, there is a problem in the opposite case, in other words, in conversion of data from MPEG-4 to MPEG-2, that motion vectors cannot always be reused.
This occurs because MPEG-2 does not support all the stipulations regarding block size, motion vectors and the like that the higher MPEG-4 standard supports. Specifically, the only block size supported by MPEG-2 is 16 pixels×16 pixels, whereas MPEG-4 also supports blocks of 8 pixels×8 pixels. Furthermore, while MPEG-2 allows motion vectors only up to half-pixel accuracy, quarter-pixel accuracy is allowed in MPEG-4.
A technique has been developed for carrying out data conversion that resolves these differences (see Japanese Laid-Open Patent Application H11-275592). A brief description thereof is as follows. Data conversion is realized by calculating one motion vector of a 16 pixel×16 pixel block from four motion vectors obtained from four corresponding 8 pixel×8 pixel blocks, then re-searching in a vicinity of the motion vector at quarter-pixel precision to find a motion vector at half-pixel precision for use.
In addition to the MPEG-2 standard and the MPEG-4 standard, an MPEG-4/AVC standard has been newly standardized (see ITU-T H.264 Standard) in which encoding processing is basically the same as in MPEG-2 and MPEG-4.
The MPEG-4/AVC standard has applications ranging from television broadcasting and the like to which MPEG-2 is directed, through to mobile, Internet and the like to which MPEG-4 is directed. MPEG-4/AVC is a scalable standard that is capable of dealing with images of QCIF (quarter CIP) size (180 pixels×144 lines) through to HD (high definition) size (1920 pixels×1080 lines), and also is able to realize an extremely high compression rate compared to the other standards.
Consequently, it is anticipated that with more wide-spread use of MPEG-4/AVC, there will be heightened demand for moving picture conversion apparatuses that convert moving pictures between the MPEG-4/AVC standard and the MPEG-2 standard, which has a high affinity with products such as conventional DVDs, and the MPEG-4/AVC standard and the MPEG-4 standard.
However, due to the-fact that the MPEG-4/AVC standard differs in part from the MPEG-4 standard, the described technique does not enable data conversion from MPEG-4/AVC to MPEG-2, and hence, does not enable a reduction of the time taken for conversion processing in such a case.